1. Field of the Invention
The present invention generally relates to a color conversion processing method for color matching and, more particularly, to a color conversion processing method for applying an appropriate color matching process to each color in a bit map image, a character image or a graphic image. The present invention also relates to a computer readable medium storing programs which cause a computer such as a general-purpose computer to perform the above-mentioned color conversion processing method.
2. Description of the Related Art
Presently, a color matching system which uses device independent colors is studied so as to provide color image data to a color printer, the color image data accurately representing a color image displayed on a color display monitor. The purpose of provision of such a color matching system is to obtain the same color image from different devices. Generally, a signal matching a visual sense of a human being such as the CIE 1931 XYZ or the L*a*b* signal is used as a color signal which is independent for each device.
However, a color representation, which is preferable to a human being, differs according to types of images such as a natural color image (photographic image), a character image or an illustration image. Accordingly, in order to obtain color image information which includes various types of images, an architecture which can switch a color processing according to a document structure included in a color image is required.
Japanese Laid-Open Patent Application No. 7-107312 discloses a color information processing method and apparatus which is related to a technique for changing a color processing in accordance with a document structure included in the color image. In the method and apparatus of this patent document, attribute information of a color matching which is appropriate for various types of images is set by application software, and a driver or a color matching processing unit in a printer performs a suitable color matching process in accordance with the attribute information.
However, in the color image outputting system disclosed in the above-mentioned patent document, there is a problem in that a matching error of a specific color cannot be prevented. The matching error refers to a phenomenon that, for example, a pure black character cannot be represented in true black in a case in which a document image includes only character images, which are in a plurality of colors, and the same color matching is applied to each of the colors. This is because, in a color image outputting apparatus such as a laser printer or an inkjet printer, a black image is formed by using a black colorant and a plurality of other colorants superimposed on the black image, and an offset of each color with respect to other colors is conspicuous. Thus, a desired pure black cannot be obtained when a color matching is applied to a line image such as a character image or a rule image when a color printer such as a laser printer or an inkjet printer is used. In order to eliminate such a problem, it is suggested that a color matching process not be applied to a specific color which tends to generate the matching error or a specific color to which a viewer is sensitive to the matching error. Alternatively, a special color matching process may be applied to such a specific color. Additionally, it is suggested that a color matching process provide a function to form a black image by a black colorant alone.
Additionally, it is difficult to develop a color matching process which can achieve an optimum condition for each color. That is, it is difficult to develop a device profile or a color conversion engine which can achieve an optimum condition for each color. Accordingly, there is a problem in that an accuracy of a color matching process varies for each color.
Further, on the output side such as a color printer, a plurality of colorants (for example, ink or toner in cyan, magenta, yellow or black) must be provided in a color image outputting apparatus, each of the colorants being a color component, and the number of colors to be provided in the color image outputting apparatus is increasing. Since the color represented by the colorants for these color components (primary colors) is controlled to be optimum, it is possible to eliminate a color matching process. That is, it is better to output a color image in the primary colors without color matching. Especially, when the color image outputting apparatus has the yellow colorant, a yellow colorant image directly output without color matching may be presented to a viewer as a clear color image.